Wormholes
Time Limit: 2000MS | Memory Limit: 65536K | |
Total Submissions: 36729 | Accepted: 13444 |
Description
While exploring his many farms, Farmer John has discovered a number of amazing wormholes. A wormhole is very peculiar because it is a one-way path that delivers you to its destination at a time that is BEFORE you entered the wormhole! Each of FJ‘s farms comprises N (1 ≤ N ≤ 500) fields conveniently numbered 1..N, M (1 ≤ M≤ 2500) paths, and W (1 ≤ W ≤ 200) wormholes.
As FJ is an avid time-traveling fan, he wants to do the following: start at some field, travel through some paths and wormholes, and return to the starting field a time before his initial departure. Perhaps he will be able to meet himself :) .
To help FJ find out whether this is possible or not, he will supply you with complete maps to F (1 ≤ F ≤ 5) of his farms. No paths will take longer than 10,000 seconds to travel and no wormhole can bring FJ back in time by more than 10,000 seconds.
Input
Line 1: A single integer, F. F farm descriptions follow.
Line 1 of each farm: Three space-separated integers respectively: N, M, and W
Lines 2..M+1 of each farm: Three space-separated numbers (S, E, T) that describe, respectively: a bidirectional path between S and E that requires T seconds to traverse. Two fields might be connected by more than one path.
Lines M+2..M+W+1 of each farm: Three space-separated numbers (S, E, T) that describe, respectively: A one way path from S to E that also moves the traveler back Tseconds.
Output
Lines 1..F: For each farm, output "YES" if FJ can achieve his goal, otherwise output "NO" (do not include the quotes).
Sample Input
2 3 3 1 1 2 2 1 3 4 2 3 1 3 1 3 3 2 1 1 2 3 2 3 4 3 1 8
Sample Output
NO YES
Hint
For farm 1, FJ cannot travel back in time.
For farm 2, FJ could travel back in time by the cycle 1->2->3->1, arriving back at his starting location 1 second before he leaves. He could start from anywhere on the cycle to accomplish this.
题意:John的农场里有n块地和m条路双向路以及w个虫洞,虫洞是一条单向路,不但会把你传送到目的地,而且时间会倒退T秒。我们的任务是知道会不会在从某块地出发后又回来,看到了离开之前的自己。
说下输入:n块地,m条边,w个虫洞。下面依次是m条边的信息(双向),输入完后是w个虫洞的信息(单向)。
思路:看图中有没有负权环,有的话John可以无限次走这个环,使得时间一定能得到一个负值。所以存在负环话就是可以,没有的话就是不可以了。
#include<stdio.h> #include<string.h> #include<queue> #define MAX 20000 #define INF 0x3f3f3f using namespace std; int n,m,ans,s; int beg,en; int dis[MAX],vis[MAX]; int head[MAX]; int used[MAX]; struct node { int u,v,w; int next; }edge[MAX]; void init() { ans=0; memset(head,-1,sizeof(head)); } void add(int u,int v,int w) { edge[ans].u=u; edge[ans].v=v; edge[ans].w=w; edge[ans].next=head[u]; head[u]=ans++; } void getmap() { int i,j; scanf("%d%d%d",&n,&m,&s); while(m--) { int a,b,c; scanf("%d%d%d",&a,&b,&c); add(a,b,c); add(b,a,c); } while(s--) { int a,b,c; scanf("%d%d%d",&a,&b,&c); add(a,b,-c); } } void spfa(int sx) { int i,j; bool flag=false; queue<int>q; memset(vis,0,sizeof(vis)); memset(used,0,sizeof(used)); for(i=1;i<=n;i++) dis[i]=INF; vis[sx]=1; dis[sx]=0; used[sx]++; q.push(sx); while(!q.empty()) { int u=q.front(); q.pop(); vis[u]=0; for(i=head[u];i!=-1;i=edge[i].next) { int top=edge[i].v; if(dis[top]>dis[u]+edge[i].w) { dis[top]=dis[u]+edge[i].w; if(!vis[top]) { vis[top]=1; q.push(top); used[top]++; if(used[top]>n)//当一个点进队列大于n次则证明存在负环 { flag=true; break; } } } } if(flag) break; } if(!flag) printf("NO\n"); else printf("YES\n"); } int main() { int t; scanf("%d",&t); while(t--) { init(); getmap(); spfa(1); } return 0; }